This paper presents numerical simulations of mixing phenomena in a double T-shaped micromixer to which periodic pressure disturbances are added to enhance the mixing efficiency. The fluids were brought in contact at the upper T-junction. The pressure disturbances of various frequencies (0-50 Hz) and phase angles were introduced through the side channels of the lower T-structure. The simulations were carried out for microchannels having the same cross-section area but with different aspect ratios (1.6-10) at low Reynolds numbers (2.9-8.8). It is found that the mixing efficiency rapidly increases with an increase of the oscillation frequency in the lower range (0-8 Hz) to reach a maximum and then decreases in the higher frequency range except for the in-phase disturbance. The mixing is enhanced most significantly as the two pressure disturbances oscillate exactly out of phase. The effects due to channel aspect ratio and inlet velocity are also discussed.